My wife and I wanted to switch services a few years ago. We compared prices and packages on Xfinity with what we had with DirecTV. The Xfinity package was cheaper (when you factor in the phone and broadband also) but only for the promotional period (24 months in our case). We were able to call DirecTV and did get a little better deal then we were paying though.
You just have to compare the packages as closely as possible and then decide.
I doubt that it is true – at least not in the long run. Sure at first there might be a deal that makes it cheaper for six months to a year but it will be associated with a contract of at least a two year period. Most telecom companies will use this maneuver to try to get new customers and entice them into believing that the new service is bigger and better for less money than any other service. This however is rarely ever true for longer than a few month period.